1. Field of the Invention
This invention relates to an apparatus and process for hot tapping of non-conductive pipes, such as polyethylene pipes, in in-service pipelines without taking said pipelines out of service.
2. Description of the Prior Art
Hot tapping of in-service pipelines is used to add valves, regulators, branch connections, and other devices to existing pipelines as well as to make in-service repairs on such pipelines. In hot tapping an in-service pipeline, it is necessary to cut an opening in the pipe for insertion of the valve or regulator, or connection of a branch pipe. To prevent fluid within the pipe from escaping, a hot tap saddle is sealingly secured to the pipe around the portion of the pipe in which the hole is to be made. The saddle is provided with a gate valve or other means for providing access to the pipe without permitting fluid to escape from the pipe. A saddle suitable for such purpose is taught by U.S. Pat. No. 4,655,480.
To make the necessary hole in an in-service pipe, it is known to use mechanical cutters. However, the use of mechanical cutters creates chips which can fall into the pipe and be entrained in the fluid stream, eventually lodging in valve seats and meter diaphragms. In addition, mechanical hot tapping equipment is necessarily large and heavy to provide the necessary cutting torque. With plastic pipe, in particular, the application of such torque is undesirable as it unnecessarily loads the pipe. Furthermore, with mechanical cutters, stress is created at the edge of a cut resulting, in practice, in openings having an area less than the transverse cross-sectional area of the hot tap apparatus through which the mechanical cutter is inserted. In addition, and more importantly, the hole shapes made by mechanical cutters are generally limited to circles. But see U.S. Pat. No. 4,270,559 and U.S. Pat. No. 2,676,604, both of which teach non-circular openings in pipes for insertion of a cylindrical flat valve member.
In plastic pipes, in particular, non-circular openings provide a substantial benefit over circular openings for hot tapping to add valves, regulators, branch connections and other devices, particularly with respect to the saddle fittings used in hot tapping. To provide the necessary joint strength between the saddle fitting and the pipe, a large amount of heat fusion surface area on the pipe around the opening is important. On the other hand, the amount of fluid passing through the fitting is proportional to the area of the opening. These requirements conflict with one another with circular openings. As the diameter of a circular opening increases to the internal diameter of the pipe, thereby increasing the amount of fluid that can flow through the opening into, for example, a branch connection, the joint formed by the saddle fitting and the plastic pipe gets weaker. Conversely, as the diameter of a circular opening decreases, thereby increasing the strength of the joint, the amount of fluid flowing through the opening decreases. The use of an elliptical opening with its major axis parallel to the axis of the pipe permits a strong joint with high fluid flow. In addition, non-circular openings permit the use of advanced flow stopping techniques, such as soft-seated gate-type stoppers which induce only minimal stresses in the low strength plastic pipe.
To provide holes in plastic material, in addition to using mechanical cutters, it is generally known to use heated devices. U.S. Pat. No. 4,204,447 teaches a punching apparatus using a heated punch to perforate a corrugated plastic pipe to create circular holes. U.S. Pat. No. 3,546,742 teaches a method of perforating flexible thermoplastic material using induction heated, thin probes. U.S. Pat. No. 4,667,552 teaches an apparatus for perforating thermoplastic film material with heated perforation pins. Finally, U.S. Pat. No. 4,449,434 discloses an apparatus for cutting holes and notches in thermoplastic films using heated steel rule dies. However, none of these devices is suitable for hot tapping an in-service plastic pipeline. In addition, none of these devices is capable of producing non-circular openings in in-service plastic pipes.
To enhance the effectiveness of mechanical cutters in machining and drilling, it is generally known to use ultrasonic vibrations as taught by U.S. Pat. No. 4,343,111 in which an ultrasonically vibrating tool is positioned across the cutting front of the work piece, the vibration resulting in removal of material from the cutting front of the work piece. Similarly, U.S. Pat. No. 4,992,639 discloses the combination of electric discharge machining and ultrasonic drilling to drill holes in metal components having a nonconductive element, in particular, a ceramic surface layer. However, the use of such vibrating tools for use in cutting holes in plastic materials, in particular, plastic pipe, is not taught by the known prior art.